In a turbo refrigerator, a small gas turbine, or the like, a rotating machine such as a centrifugal compressor is provided. The centrifugal compressor includes an impeller in which a plurality of blades are provided on a disk part fixed to a rotary shaft. In the centrifugal compressor, pressure energy and speed energy are applied to the gas by rotating the impeller.
For example, when a light fluid such as hydrogen is compressed, or when a higher supercharging pressure is required, or the like, it is necessary to rotate the impeller of the centrifugal compressor at a high speed. More specifically, for example, when hydrogen is compressed, like a case where the number of rotations of the impeller increases from several thousands rpm to several tens of thousands rpm, it is necessary to rotate the impeller at a high speed. Particularly, in a centrifugal compressor in which a rotary shaft is inserted into an attachment hole formed in a center portion in a radial direction of the impeller and the entire inner peripheral surface of the attachment hole is gripped by the rotary shaft, when the impeller is rotated at a high speed, tensile stress in the vicinity of the inner peripheral surface of the attachment hole increases, and thus, the attachment hole may be damaged.
Accordingly, in order to prevent the tensile stress in the vicinity of the inner peripheral surface from increasing, it is suggested that a stress reduction depression is formed on the inner peripheral surface of the attachment hole (for example, refer to PTL 1).